Comparison of different metal sulfate catalysts
In this study, the role of a series of metal sulfate catalysts on the oxidative lignin depolymerization was evaluated preliminarily. The effect of H2SO4 on the oxidation of lignin was considered in the present of CuSO4 (point c in Figure 3), as it has reported that BrĂ²nsted acid is an effective promoter for lignin oxidation.43 Experimental results exhibit that the existence of H2SO4(12.5 mmol L-1) in the microemulsion system does promote the oxidation of lignin and enhance the yield of phenol monomers (61.9 to 89.3 mg g-1, Table 3, entries 1 and 4). Therefore, the above microemulsion containing 12.5 mmol L-1 H2SO4 was used while other cheaper and water-soluble metal sulfates with divalent cations (CuSO4, MgSO4, ZnSO4 and FeSO4) were used with consideration to the salt effect of the microemulsion (Table 3, entries 2-6). The yield of phenolic monomers is 69.2 mg g-1 in the presence of FeSO4, but much higher yields are provided by CuSO4 and ZnSO4 (89.3 and 88.9 mg g-1 respectively), while the highest yield ofp -hydroxy benzaldehyde (32.2 mg g-1) is achieved in the presence of CuSO4 (Table 3, entries 4 and 5). These differences between metal sulfates can be attributed to the different redox potential of metal cations.44However, there is negative effect for lignin oxidation in the acidic condition under the catalysis of MgSO4, because it belongs to alkali salt (Table 3, entry 6). Besides, the influence of metal sulfates with monovalent and trivalent cations (Na2SO4 and Fe2(SO4)3) to lignin oxidation was also investigated (Table 3, entries 7 and 8). But they both show much lower catalytic activity than that of CuSO4. It should be noticed that the yield of phenolic monomers in the present of Fe2(SO4)3 is lower than that of FeSO4, which will be oxidized completely to Fe2(SO4)3 at the atmosphere of O2 (Table 3, entries 3 and 8). This difference can be considered to trigger more serious peroxidation of products under the catalysis of Fe2(SO4)3.
Table 3. Comparison of metal sulfates on lignin oxidation in the microemulsion.